TEE SCIENTIFIC INVESTIGATION OF CANCER 213
mainly by the organization of the animal and at a place not accessible to a direct experimental change. The external factors consist of con- ditions operative in the environment of the cells which give origin to cancer, either within or without the whole organism of which the cancer cells form a part. The external factors are more accessible to experi- mental control than the internal. The external factors which set np changes in the cells and tissues are usually only the first link in the chain of circumstances which ultimately ends in the production of cancer. And this link we may generally call a stimvivs.
Let us then first consider some of the internal factors and later the external factors which lead to cancer growth. One of the most important internal factors which has been thoroughly established within recent years, at least in the case of animal cancer, is heredity. Our at- tention was first drawn to the possibility that heredity might play a part in the origin of cancer, through observations which we as well as other investigators, made some eighteen years ago. It was found that a surprising multiplicity of cancers occurred in certain cages in which animals were kept, especially rats and mice, and that the varieties of cancer which appeared under those conditions were in each case iden- tical. This led to the conclusion that cancer was so often found in those cages because the animals living there belonged to the same family.
There are other possible explanations for the frequent occurrence of cancer in animals kept in the same cage or breeding place. It might have been that the cancer spread through infection from one animal to another. Indeed, in the case of man, the alleged greater frequency of cancer in certain regions has been explained by some observers on the basis of the infectiousness of cancer. However, I may state here that no undisputed case, in animals or in man, has ever been recorded where cancer was known to have spread through infection caused by a micro- organism. This lack of infectiousness, however, does not entirely ex- clude the possibility of the presence of microorganisms in certain can- cers — ^a possibility to which I shall have occasion to refer later. There are certain cases in which metazoic parasites are undoubtedly responsible for the so-called " endemic ^^ occurrence. We have, however, been able to decide the question as to the usual cause of the occurrence of identical cancerous growths in animals by showing that hereditary conditions play a paramount r51e. For this purpose we used mice, which are per- haps most suited to such experiments, since they are not long-lived ani- mals. We kept them in families separated from each other throughout a number of years, but otherwise absolutely under the same conditions as far as climate and food were concerned. Now we found that each family had a definite cancer rate which remained approximately corir slant throughout succeeding generations. While in some strains cancer
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